1. Field of the Invention
This invention relates to a thermal spray process which uses solution precursors as a feedstock.
2. Description of the Related Art
Coatings are commonly used to provide desirable surface properties of the underlying bulk substrates. Examples of protective coatings include wear-resistant, corrosion-resistant and thermal barrier coatings. In many applications, multiple properties of the coatings are often desirable. However, it is not always possible to have a single material (single phase material, alloy or composite) that has all the required properties. In such a case, multiple materials with different properties can be used in the form of multilayers.
Conventional coatings, including multilayered coatings, are made of coarse-grained materials with grain sizes which are greater than several microns. These coatings can be prepared by solution chemistry, physical or chemical vapor deposition or thermal spraying. For deposition methods that do not involve solution based chemistry, physical vapor methods such as sputtering and beam induced evaporation are commonly used. The vapor of the materials (as atoms or clusters) condense on the substrate to form coatings. The chemical vapor approach generally involves pyrolysis of chemical precursors at the substrate to form desirable reaction product coatings. Vapor techniques are generally suitable for preparing thick films or thin coatings due to the low rate of deposition.
An alternative approach to fabrication of thick coatings is thermal spraying. In thermal spraying powders are generally used as the feedstock and fed into a flame aimed at the surface of substrates. The powders are propelled in the gas flow and are fused to form coatings on the substrate. Thermal spraying includes plasma methods in the ambient atmosphere or vacuum, high velocity oxyfuel spraying or high velocity impact fusion spraying. In all cases, the feedstock are often very coarse agglomerates of powders. The agglomerate size is typically in the tens of microns. The powder agglomerates often form splat microstructures, which are pancake-like structures in the thermally sprayed coatings.
Although thermal spraying is a viable approach to preparing thick coatings, the use of the powder agglomerate feedstock has limitations and problems. First, the sprayable powders often require reprocessing from the parent powders by controlled agglomeration, which adds more cost to the production and often introduces impurities if surface-active precursors are used as binders. Second, the splat boundaries in the as-sprayed coatings are often the initiation sites for flaw propagation that consequently lead to mechanical failure of the coatings. Third, the as-formed splat microstructures present a limitation on the scale of chemical homogeneity and mixing of multiphasic materials when desired because the splat is at least greater than several microns thick, due to the flattening of the molten particles on impact. From commercial experience, sprayable powders need to be of a certain size such as about 30 microns or larger for efficient deposition. As a result, reconstitution of nanoscale powder to 30 micron-sized agglomerates is often required. Unfortunately, these larger diameter agglomerates produce longer splat microstructures in the coating. These large splat particles become a serious problem when multifunctional applications require multilayered, hybrid coatings with fine, continuous interfaces, since the length scale of an interface is limited by the splat microstructure.
To solve this fine gradient coating problem, we proposed to use liquid solutions wherein the composition of the solution is varied as the coating is applied. Although it has been known to use a liquid feedstock in thermal spraying, such disclosures do not relate to the production of nanostructure coatings and the multilayer and gradient coatings of the present invention.
U.S. Pat. No. 5,032,568 to Lau et al uses an atomized aqueous solution containing at least 3 metal salts precursors into an inductively coupled ultra high temperature plasma for coating. There is no discussion of forming nanostrucure coatings nor of how to provide multilayer and gradient coatings on such a small scale.
U.S. Pat. No. 4,982,067 to Marantz et al relates to an apparatus to eliminate the long-standing problems with radial feed plasma spray apparatus by designing a true axial feed in a plasma spray system. While most of this disclosure is to using particles as the feed, at column 5, lines 51-55, the patent states that xe2x80x9calternatively, the feedstock may be in liquid form, such as a solution, a slurry or a sol-gel fluid, such that the liquid carrier will be vaporized or reacted off, leaving a solid material to be deposited.xe2x80x9d Again, there is no discussion of forming nanostructure coatings nor of how to provide multilayer and gradient coatings on this small scale. In addition this patent essentially deals with the deposition of solid particles that are formed by conversion of the droplets to solid particles in flight before impacting the substrate.
U.S. Pat. No. 5,413,821 to Ellis et al relates to an inductively coupled plasma to thermally decompose a chromium bearing organometallic compound. Column 2, lines 19-22, states that the organometallic compound can be introduced to the plasma as a vapor or a solid. However, in Example 4 the tetramethylchromium is cryogenically cooled to the liquid state for application to the plasma coating device. The organometallic liquid was introduced into the plasma by bubbling through a carrier gas or in the form of solid powder entrained in the carrier gas. The former may actually exist in the form of chemical vapor. Again, there is no discussion of forming nanostructure coatings nor of how to provide multilayer and gradient coatings on this small scale.
U.S. Pat. No. 5,609,921 to Gitzhofer et al discloses a suspension plasma spray where a suspension of particles of the material to be deposited is in a liquid or semi-liquid carrier substance. An inductively coupled radio-frequency plasma torch is used. The preformed particles are suspended in a liquid carrier. Vaporization of the liquid carrier in the plasma leads to the agglomeration of the particles. The particles become molten and impact the substrate. Suspension of small particles in a liquid and its subsequent spraying into the plasma flame may lead to an additional problem. If the particles are dispersed and are very fine (such as less than 100 nm), they may not have enough momentum to penetrate into the plasma flame and be carried by the plasma flame to the substrate. Again, there is no discussion of forming nanostructure coatings nor of how to provide multilayer and gradient coatings on this small scale.
It is an object of this invention to use of solution precursors as feedstock in thermal spraying of ceramic, metallic, organic and hybrid (a combination of various classes of materials) coatings in a potentially competitive, single step fabrication process for coatings.
It is a further object of this invention to provide a thermal spray process which eliminates the need to synthesize powders and reprocess these powders for spraying.
It is a further object of this invention to utilize the chemical conversion of droplets in a thermal spray process to form desirable reaction products as coatings on substrates in a single step synthesis process.
It is a further object of this invention to provide a thermal spray process in which solution feedstocks are employed for better homogeneity and mixing of multiphasic materials.
It is a further object of this invention to reduce costs for preparing coatings of high melting temperature materials by replacing the melting of the powder required in conventional thermal spray of powder feedstock with the lower temperature solidification of thermally sprayed droplets at the substrate.
It is a further object of this invention to utilize the molecular design of solution precursors for desirable reaction products in a thermal spray process.
It is a further object of this invention to utilize an external energy source simultaneously during the thermal spraying to affect the molecular design, structure, microstructure and interfaces of the coating.
It is a further object of this invention to utilize a post deposition application of an external energy source to further affect the molecular design, microstructure and interfaces of the coating.
It is a further object of this invention to employ a thermal spray process with a solution feedstock in which the droplet size is varied.
It is a further object of this invention when using a thermal spray process with a solution feedstock to further reduce droplet size by placing a fine screen mesh between the spray nozzle and the substrate.
It is a further object of this invention when using a thermal spray process with a solution feedstock to control the residence time, the in-flight temperature of droplet, and the working distance to the substrate to control the structure and the microstructure of the deposited coatings.
It is a further object of this invention to employ a thermal spray process in which fine droplets are allowed to solidify before reaching the substrate by controlling the in-flight temperature so that the resulting splat will have a smaller dimension compared to that obtained by using a powder feedstock.
It is a further object of this invention to employ a thermal spray process in which droplets are allowed to reach the substrate in the liquid state so that solidification of droplets at the substrate will also lead to finer splat microstructure and better chemical mixing when more than a single phase of materials are sprayed.
It is a further object of this invention to employ a thermal spray process that is suitable for producing multilayered materials that require fine scale grading, both compositionally and microstructurally, and particularly for nanostructured graded materials.
It is a further object of this invention to employ a thermal spray process which permits the integration of layers by gradually graded interfaces rather than abrupt interfaces so as to permit the compatibility of hybrid multilayered materials, i.e. ceramics-ceramics; metal-ceramics; metal-metal, organic-inorganic; and in any combination.
It is a further object of this invention to employ a thermal spray process which permits microstructural, structural and chemical grading with continuous interfaces at a fine scale.
It is a further object of this invention to employ a thermal spray process in which the process can be adapted to contain nanostructured preformed particles in solution so that the solution provides the percolating matrix whereas the powders provide the major constituents of the coating layers.
These and further objects of the invention will become apparent as the description of the invention proceeds.
It has now been found that thin films or coatings can be made of nanostructured particles which have a particle size less than 100 nm (i.e. 0.1 micron) by thermally spraying a solution of a liquid coating precursor feedstock onto a substrate to form the film or coating. The resulting thin film or coating has a thickness of about 100 nanometers or larger. By using thermal spraying with different precursor feedstock solutions, coatings can be made with more than one layer. Within a given layer, by varying the composition of the precursor feedstock, a composition gradient coating can be formed having nanoparticle size particles of less than 100 nm. Many combinations of materials can be co-deposited, such as ceramics-ceramics, metal-ceramics, metal-metal, and organic-inorganic. By applying an external energy source either during the coating process or during post deposition, the resulting coating can be modified.
A further feature of the invention is that multifunctional, multilayered, nanostructured coatings can be better prepared by using solution feedstocks in the thermal spray deposition process. This permits tailored engineering of the interfaces at a finer length scale by compositional and microstructural grading throughout the entire coating thickness. This process permits an efficient conversion of molecules-atoms (solution dependent) into aerosol droplets and subsequent chemical reactions to form the product layers on the substrate. With post-deposition treatment of the as-synthesized coating, there can be optimized microstructures, structures, density and adhesion.
By using thermal spraying of solution precursor feedstocks, compositionally and microstructurally graded coatings are fabricated which have unique advantages. The molecular level mixing of the constituents in solution precursor feedstocks allows for better chemical homogeneity of sprayed products. By using fine droplets that are many times smaller than the conventionally used powder feedstock (e.g. 30 microns or larger in particle size), a finer scale of microstructure can be achieved. The solidification of droplets can be controlled in flight or on impact on the substrate by controlling the spray temperature, the working distance and the substrate temperature. This provides a means to reduce the size of microstructure as compared to the powder feedstock routes. Finally, functional grading of multilayered coatings can be achieved at a much finer scale, particularly for nanostructured graded coatings, both compositionally and microstructurally, compared to the powder feedstock approach wherein the size of splat poses a limit on the scale of mixing and grading. Functional grading may include, but is not limited to, the graded continuous interface where the microstructure, structure and chemistry of two or more materials are varied continuously. Such grading may enhance the thermal, chemical and mechanical stability of multilayerd coatings and the control of the mechanical, electrical, magnetic and other transport properties.